The prior art is described with particular reference to the field of nuclear medicine imaging, where the present invention is principally, although not exclusively, applied.
Nuclear imaging aims to estimate the spatial, spectral and temporal distributions of a radioisotope by detecting the radiation of the object. One particular detection device used for nuclear medicine imaging is described in U.S. Pat. No. 3,011,057 to Anger. This device analyzes the response of a set of photomultiplier (PM) tubes in order to estimate the energy and the position of a scintillation. Each PM response is proportional to the light created during the scintillation (function of the gamma energy) and to the solid angle subtended by its photocathode (function of the depth-of-interaction, DOI). Since the DOI is usually not available to the estimation process, the mean response is therefore used, implicitly or explicitly, as an approximation. For a given event, deviation from mean parameters will inevitably create errors in the estimation procedure.
The stochastic nature of the gamma ray interaction with the crystal and the fact that only the mean depth-of-interaction is accessible by external measurements, result in non-uniformities and distortions in the detection field. Correction of these non-uniformities has received a lot of attention in the past and different complementary and competitive technologies have been described. U.S. Pat. Nos. 3,745,345 to Muehllehner, 4,142,102 to Lange, 4,228,515 to Genna, 4,316,257 to Del Medico, and 4,475,092 to Arseneau are of particular interest in this context.
While the hereinbefore mentioned methods of the prior art generally perform an adequate mean correction, compensating for the- mean observed distortion both in the spatial and the energy domains, they do not provide optimal correction on an event-by-event basis for they do not take into account the event DOI largely responsible for the variation in these signals.
Methods making explicit use of the DOI in the detection process have already been proposed. Cook et al in "A Thick Angle Camera for Gamma-Ray Astronomy" IEEE Trans. Nucl. Sci. vol. NS-32, pp. 129-133, 1985 proposed to use two or more energies to sample the DOI in the crystal as the mean depth-of-interaction varies with the energy. This approach offers a definite gain over the standard technique but first, the sampling of DOI so obtained is relatively coarse and, second, the sampling is not perfect, as for each individual energy, the detector still integrates the response over all possible DOIs, in other words, this approach is still limited by the fact that the mean response over the DOI and the response at the mean DOI are not equal. A second approach has been proposed by Karp and Daube-Witherson in "Depth-of-Interaction Determination in Nai(TI) and BGO Scintillation Crystals Using a Temperature Gradient" Nuclear Instrumentation and Methods in Physics Research, vol. A260, pp.509-517, 1987. This technique offers the potential of estimating the DOI on an event-by-event basis but can be extremely difficult to realize for it requires a temperature gradient to be applied on the crystal so that the crystal relaxation time is modified. Amplitude of this gradient has to be constant and large enough to produce sufficient modification of the detection signals as the DOI varies.
Rogers and Saylor in U.S. Pat. No. 4,675,526 entitled "Method and Apparatus for 3-D Encoding" described a methodology by which DOI can be obtained directly by signal processing and proper hardware configuration. In addition to restriction in the light distribution, imposed by the fiber optics light guide, the authors state that at least three samples must be taken on the narrowest light spread distribution so that samples must be taken at an area interval of 1 cm or less in two dimension. A summing network (lines and columns) decreasing the burden of the computation in a typical camera of at least 1000 cm.sup.2 is then described. The sum of adjacent non-zero lines (on each side of the maximum line) is used as an indicator of the DOI. In fact, it is easily shown that this operation is equivalent to create the square root of the moment of order 2 to be described below. (The use of higher order moment as an indicator of the light dispersion has also been recognized by Yamashita et al. U.S. Pat. No. 4,945,241 in a different context). Elimination of the long tail of the light distribution through fiber optics device in the Roger's proposal is useful to decrease the noise in the calculation only if the access to individual photodetector response is not possible. Indeed, access to individual responses renders the filtering operation obsolete since selection of a cluster of tubes can always be used to eliminate unwanted (noisy) responses. Moreover, the filtering scheme decreases the quality of the total energy signal since the light distribution is not more concentrated but only chopped.
In the present invention, a method for normalizing the conventional photodetector output with respect to the DOI is provided. The method can be implemented in the conventional crystal-photomultipliers arrangement without modification and without the expense of having to put a large number of smaller detectors and light shaping devices. Furthermore, because this invention can be considered as an independent extra step in the prior art (with total energy, moments or centroids in X and Y as the final result), the rest of the chain, including the actual positioning, the correction system as described above can still be used, also without any modification. The object of this invention is to improve the energy resolution and to produce energy independent signals for positioning.